linux-kernel - Re: [RFC] [PATCH 1/7] User Space Breakpoint Assistance Layer (UBP)

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Message-ID: <20100118171038.GB4424@in.ibm.com>
Date:	Mon, 18 Jan 2010 22:40:38 +0530
From:	Ananth N Mavinakayanahalli <ananth@...ibm.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Peter Zijlstra <peterz@...radead.org>,
	Jim Keniston <jkenisto@...ibm.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	utrace-devel <utrace-devel@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Maneesh Soni <maneesh@...ibm.com>,
	Mark Wielaard <mjw@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] [PATCH 1/7] User Space Breakpoint Assistance Layer (UBP)

On Mon, Jan 18, 2010 at 06:52:32PM +0200, Avi Kivity wrote:
> On 01/18/2010 05:43 PM, Ananth N Mavinakayanahalli wrote:
>>>
>>>> Well, the alternatives are very unappealing.  Emulation and single-stepping
>>>> are going to be very slow compared to a couple of jumps.
>>>>        
>>> So how big chunks of the address space are we talking here for uprobes?
>>>      
>> As Srikar mentioned, the least we start with is 1 page. Though you can
>> have as many probes as you want, there are certain optimizations we can
>> do, depending on the most common usecases.
>>
>> For eg., if you'd consider the start of a routine to be the most
>> commonly traced location, most routines in a binary would generally
>> start with the same instruction (say push %ebp), and we can refcount a
>> slot with that instruction to be used for all probes of the same
>> instruction.
>>    
>
> But then you can't follow the instruction with a jump back to the code...

Right. This will work only for the non boosted case where single-stepping
is mandatory. I guess the tradeoff is vma space and speed.

Ananth
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